The present invention relates to data encoding and decoding systems and in particular to data encoding and decoding systems for frequency-shift key (FSK) modulation and methods therefor.
It is important in a communication system that the same data clock be available to both the transmitter and the receiver of data sequences. To that end, data clocks having a desired frequency and phase relationship can be generated simultaneously in the transmitter and in the receiver, or the data clock can be transmitted from the transmitter to the receiver. Complications arise when it is necessary or desirable to use any of a number of clocks that might be supplied to the transmitter. Likewise, a problem exists when it is inconvenient or undesirable to transmit the data clock separately from the data, as may be the case in covert communications systems. In either case it is desirable to be able to retrieve the data clock from the data stream.
One approach to retrieving the data clock from the data stream involves Manchester encoding wherein each data bit is encoded into two opposite symbols. In view of the fact that two symbols are used for each data bit, a Manchester encoded FSK transmission must have twice the bit rate as an un-encoded transmission in order to transmit the same amount of data in a given time. In as much as bandwidth is related to time because it expresses frequency, doubling the bit rate by using Manchester encoding requires doubling the bandwidth.
Although the transition at the center of every Manchester encoded bit provides a means to recover the data clock from the data stream, there are drawbacks. For example, the data IF channels must have twice the bandwidth of the data, imposing three dB more noise on the detection process. Furthermore, there is an inherent clock phase ambiguity of pi, which can be resolved only after the reception of a number of data transitions. Before resolution, the data may be (with 50% probability) ambiguous.